Most Motor Cruisers and Yachts suffer from some compass deviation. Deviation is the problem of compass error owing to the structure of the vessel, proximity of metal objects, magnetic fields or electrical equipment. Deviation also varies with vessel’s heading as this changes the position of the magnetic fields in a boat relative to the earth’s magnetic field.

Motor Cruisers tend to suffer from deviation more than yachts because so often the compass is situated far too close to electrical equipment. On a good quality compass a professional compass adjuster can almost eliminate error caused by deviation by inserting magnets into small cylinders situated round the outside of the compass. However, even good quality expensive motor cruisers tend to have a cheap low quality compass, so this type of adjustment cannot be done.

In order to allow for deviation in your navigation it is necessary to ‘Swing the Compass’, this expression tends to conjure up in my mind a picture of someone having tied a short length of rope to their compass then swinging it round their head! This is obviously not the case, but it does involve creating a ‘deviation card’ that will show the difference between each of the eight magnetic cardinal and inter-cardinal points and the actual heading shown by the boats compass.

The simplest way of achieving this is by the helmsman or woman moving the boat round so that the boats compass shows exactly 000º, 045º, 090º, 135º, 180º, 225º, 270º and 315º. Then whilst holding the boat on each course a second person should look through a hand-bearing compass straight ahead from the centre of the boat and over the stem and call out the magnetic heading. Providing the hand-held compass is used well away from any magnetic influences on the boat it will give you an accurate magnetic heading.

A third person will write down a list of the boats compass headings and alongside each of these write down the magnetic heading as called out by the second person. Where the boats compass bearing is greater then the deviation is west, if the boats compass bearing is less then the deviation is east. This can be plotted as a deviation curve on graph paper as show in the diagram to the left.

Total compass error is the combination of both deviation and variation (variation is the difference between true north and magnetic north and in the Algarve magnetic north is at present 3º west of true north). Deviation is always relative to magnetic north and as such ‘Like names add, unlike names subtract’. For example: a variation of 15º east and a deviation of 5º west gives a total compass error of 10º east. Whereas, a variation of 4º west and a deviation of 3º west gives a total compass error of 7º west.

Swinging a compass and making out a deviation card is looked upon by some as being a ‘black art’! It isn’t, in fact its lots of fun as can be seen in the picture above when I did it a few weeks ago with the owner of ‘Southern Promise’, an Ocean Alexander 56. Four of my students who were doing a course with me that week can be seen in the photograph at the top of this page helping with this operation.

PS - I wrote the article above in January 2007 and since then have had several requests from owners of yachts and motor cruisers in Portugal to swing their compass and supply a deviation card. I am not a professional compass adjuster and whilst the method described above is good enough for me, it is not the way that a professional would do it. I have for several years been looking for a compass adjuster in Portugal, but until recently have not been successful. I have in the last week (May 2010) found and been in touch with Captain J.J. Rocha Ramos who iis a retired merchant marine officer and a compass adjuster.

His company is:  NavInspect - Ship Service Ltd.  Rua do Alvito 112, R/C 1300-061 Lisboa, Portugal. Email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it   Mobile: 00 351 919982386  Tel/Fax: 00 351 213644535